These studies are concerned with the subunit structure of embryonic muscle myosin and the relationship of this structure to its biological functioning. Embryonic myosin, deficient in light chain LC3 (16,000 mol. wt.), is subject to proteolysis on storage whereas the adult form is not. Leupeptin, a low-molecular-weight proteinase inhibitor, prevents this proteolysis. A search for the responsible protease is being carried out in the hope that some information regarding protein turnover will be forthcoming. In addition, whether the absence of LC3 renders embryonic myosin more susceptible to proteolysis is under investigation.